When playing a double stop, you play two different notes together. How do you know which note it produce? Example: On guitar, playing the B on third string and G sharp on first string. Does it produce a B or a G sharp? thanks
It produces both notes. It sounds to me like what you think happens is the two notes combine together to produce one note (maybe I am misunderstanding). What actually happens is the two notes sound out at the same time, and the combination of those two notes is what you are hearing. The two notes sounding out together produce a harmony.
That's a chord... A skeleton of a chord, those two notes can fit many chords though mainly that would fit as a B Major chord, with B as root and G# as 6th IIRC. Someone with more theory knowledge may extend ;-)
As noted above it would produce both notes. Depending on what other instruments were playing (if any) would define the chord sound you would get (i.e., if a bass player played an E, you would get an E major sound. Alone, they will probably sound to most people like a g minor chord as you will have the root and third.